A Disco Ball in Space

Students and scientists are teaming up for a far-out
experiment to monitor the outer reaches of Earth`s atmosphere.

October 9, 2001: Normally when
a spacecraft blasts off for Earth orbit, ground controllers hope
it will stay up for a long time. But last month NASA launched
something a little different: a satellite whose mission is to
fall from the sky. Returning to Earth out of control and with
no hope of rescue is exactly what it`s supposed to do!

Its name is Starshine 3, and it looks as curious as its mission
sounds. The 200-lb satellite, carried aloft from Alaska on Sept.
29th by a Kodiak Star rocket, is a meter-wide sphere studded
with 1500 student-built mirrors. Sky watchers can easily see
it as it glides overhead, spinning and glittering like an oversized
disco ball.

"Starshine 3 is on a mission to explore the outer reaches
of Earth`s atmosphere and to discover what happens to satellites
there," explains Prof. Gil Moore, the director of Project
Starshine -- a unique program that combines cutting edge research
with educational outreach.

"Project Starshine is a consortium
of volunteers -- universities, government agencies, private corporations
-- led by the US Naval Research Laboratory and the Space Grant
Program," he says. The most important participants, though,
are students. Approximately 40,000 of them from all parts of
the world helped polish Starshine 3`s distinctive mirrors. And
now that the "disco ball" is in space, students are
going to help again by monitoring Starshine 3 as it falls ....
at first slowly, then later with greater haste ... back to Earth.

Starshine 3 is falling because the atmosphere is dragging
it down.

The satellite is orbiting 470 km above Earth in a region scientists
call the thermosphere. The air at that altitude is very thin
-- about 1012 times less dense than the air at sea
level. Indeed, it seems more like space than a part of Earth.
The thermosphere is where the International Space Station (ISS),
the space shuttle, and many other satellites orbit.

Even though the thermosphere is practically vacuum-thin, it`s
still dense enough to sap orbital energy from satellites by means
of aerodynamic drag. Soon after the Kodiak Star disgorged it,
Starshine 3 began to lose altitude -- dropping a few meters during
each 90 minute orbit around our planet. The same thing happens
to the ISS, which requires periodic re-boosts.

Above: Starshine 1, a beachball-sized cousin of Starshine
3, vaporized in Earth`s atmosphere last year. These data, courtesy
of NRL`s Judith Lean, show how the satellite fell slowly at first,
then with greater rapidity as it descended into lower (and denser)
layers of Earth`s atmosphere.

"Starshine 3 will slowly descend during the next 4 years,"
says Moore. As it sinks into ever denser layers of the atmosphere,
its rate of orbital decay will accelerate. Eventually, when it
sinks below the stratosphere, Starshine 3 will burn up completely.
"The end will be spectacular," he added. If re-entry
happens at night, the dazzling fireball would cast enough light
for onlookers to read a newspaper!

There`s no danger to anyone on the ground, Moore notes. "We
designed the satellite so that it will be 100% consumed about
80 km up." Except for a few small steel screws the body
of the spacecraft is made entirely of aluminum -- a substance
that will vaporize during the fiery descent. "We had no
choice," quips Moore. "Otherwise I was going to have
to buy a 100 million dollar insurance policy with a $50,000 premium."
The aluminum was cheaper.

No
one knows exactly when or where Starshine 3 will return because
the thermosphere is unpredictable. Earth`s uppermost atmosphere
"breathes" in and out in response to changing solar
activity. During years (like 2000 and 2001) around the peak of
the 11-year sunspot cycle, our star is a powerful and sometimes
fitful source of extreme ultraviolet (EUV) radiation. Waves of
EUV heat the thermosphere and cause it to "puff
up," hastening the descent of falling satellites. Exploring
the cause and effect relationship between solar activity and
orbit decay is a primary goal of the mission.

"We have very few direct measurements of the density
of the upper atmosphere. That`s why Starshine 3 is so important,"
explains solar physicist Judith Lean of the Naval Research Laboratory
(NRL) in Washington, D.C. "The satellite`s well-defined
spherical geometry allows us to estimate its ballistic coefficient
with a reasonable degree of certainty. As a result, by monitoring
Starshine`s orbit and studying how that decays we can calculate
the density of the gas that`s dragging it down."

Lean and her colleagues, led by NRL`s Mike Picone, are already
using data from an earlier mission (Starshine
1, which orbited Earth for 8 months before it disintegrated
in February, 2000) to predict the decaying trajectories of satellites
and bits of orbiting space debris. Their calculations are of
keen interest to ISS mission planners and shuttle pilots who
fly through the swarm of Earth-orbiting objects. "We`re
hoping that Starshine 3 will improve those predictions even more,"
says Gil Moore.

Before
that can happen, though, someone must track the glittering satellite.
Moore and his colleagues need to record about 800 sightings per
day to define Starshine 3`s changing path.

It`s the perfect job for a legion of young scientists.

"We need students, teachers -- sky watchers of all kinds
-- to go outside and spot Starshine 3 among the stars,"
says Moore. "The procedures for recording and reporting
the data are straightforward. Anyone can do it. It`s a great
science project and we hope many classrooms will sign
up to participate."

Above: Prof. Gil Moore holds a full-scale mockup of
the Starshine 1 and 2 satellites. Sunlight is reflected from
a single mirror in the image on the right. Photo by Kerry Kirkland.

Moore added: "Starshine 3 usually looks as bright as
a 1st magnitude star when it passes overhead." During a
typical transit the rotating satellite will seem to flash every
few seconds as sunlight glints off one mirror after another.
The trail of bright dots and dark dashes across the sky is unmistakable.

"It`s beautiful," says Sharon Simon, a teacher at
the Davis Creek Elementary School who saw the satellite glide
over her town in West Virginia on October 2nd. "The flashes
came in three quick segments that were clearly visible. [Each
flash] was about as bright as Mars." (Visit Heavens-Above.com
to find out when Starshine 3 will glide over your town,
suggests Moore.)

Simon`s
students helped polish Starshine 3`s mirrors, and their handwritten
signatures are orbiting Earth on board the satellite. Indeed,
the signature of every student who worked on the mirrors
is up there -- all 40,000 of them laboriously digitized by Moore`s
wife Phyllis. "That was hard work for Phyllis," recalls
Moore. "But these youngsters are important to us -- they
are a crucial part of what we`re doing."

Moore hopes that thousands more students will join the program.
"We need more spotters," he says -- not only to monitor
Starshine 3, but also to track Starshine 2, another "disco
ball" slated for launch aboard space shuttle Endeavour in
December 2001. Starshine 4 and Starshine 5 are on the drawing
board, too, and they will need mirrors as well. Schools should
soon be able to request mirror-building kits for those satellites
at the Project Starshine
web site.

"Our goal," says Moore, "is to monitor the
thermosphere during all phases of the 11-year solar cycle using
Starshine satellites launched every year or so." If his
plan succeeds, watching disco balls glide across the heavens
might soon become a regular part of K-12 science classes. Who
knows ... perhaps disco isn`t dead after all.

Heavens-Above.com -- this website to find
out when Starshine 3 will glide over your town.

NASA`s J-Pass -- Another online resource
to help you spot Starshine 3 from your own back yard.

Protecting Starshine -- All of Starshine 3`s student-polished
mirrors were coated with a layer of silicon dioxide. "The
silicon dioxide coating adds no reflectivity," says Vince
Huegele, "but it does protect the mirrors from corrosion
by atomic oxygen [a chemical constituent of the thermosphere]."
Huegele works at the Marshall Space Flight Center`s Space
Optics Manufacturing Technology Center where the essential
coating was applied.

Solar S`Mores -- (Science@NASA) As a result
of the solar maximum, Earth`s atmosphere is "puffed up"
like a marshmallow over a campfire leading to extra drag on Earth-orbiting
satellites.

Right:Layers
of the Earth`s atmosphere. The troposphere is the first
layer above Earth`s surface; it contains half of our planet`s
atmosphere. Weather occurs in the troposphere. Many jet aircraft
fly in the stratosphere because it is very stable. The stratosphere
contains the ozone layer. Meteors burn up in the mesosphere.
Auroras glow in the lower thermosphere; satellites like
Starshine 3 and spacecraft like the Space Shuttle orbit in the
upper thermosphere.